The present invention relates generally to a color xerographic system, and, more particularly, to a system which incorporates a photoreceptor belt having registration marks on the surface thereof whose location must be precisely detected for image registration purposes.
High speed copiers and digital printer machines typically employ a photoreceptor belt as the imaging media since the belt can provide significantly more area to form a plurality of latent images during a single pass or revolution when compared to a machine with a photoreceptor drum as the imaging surface. In a color copier, a plurality of image exposures are formed, developed and transferred to an output paper sheet in either a single or multiple pass mode, to form a composite color output image. The position of the belt during operation must be known with a great deal of precision since the system timing control must ensure that the images are formed within preselected frames on the photoreceptor surface and each subsequent image is superimposed (registered) with previously formed images. The image exposures must be coordinated with the development, transfer and paper feeding functions. Registration errors occur because the belt is subject to changes in physical size and its rotational speed may vary because of mechanical wear and mechanical tolerances in the drive components. These errors are manifested in the output copies which may exhibit color separation, color bleeding and/or other defects which make the output copies unsuitable.
Various techniques have been developed in the prior art to compensate for these errors. One method is to form registration marks on the surface of the belt, either at manufacture or by exposing the belt in non-image areas to form the mark. The latent image of the mark can then be sensed or, alternatively, the latent image of the mark can be developed and sensed in conjunction with an illumination source.
The following patents contain disclosure of techniques for sensing marks placed on the photoreceptor belt for various purposes.
U.S. Pat. No. 4,912,491 discloses an apparatus for forming superimposed images and registration marks corresponding to the position of the images associated therewith. The registration marks are formed apart from the imaging portion of the medium in a transparent area to be illuminated from the opposite side of the belt. Detectors sense the position of the registration marks as the marks pass between the illuminated areas. The sensing of the registration marks is used in determining proper registration positioning, whereby the image forming devices may be adjusted to achieve such registration.
U.S. RE. Pat. No. 32,967 discloses a web tracking system for a continuous web which passes along a predetermined path through one or more processing stations. The tracking system has aligned tracking indicia on one or both sides of the web and detectors sense these indicia which are indicative of dimensional changes in width and length of the web at a particular point. An edge sensor is also provided to determine movement of the web.
U.S. Pat. No. 5,175,570 discloses a color image forming apparatus which forms registration marks la, lb, either as holes formed in the belt or as marks formed on the belt surface. The marks are then sensed and used to adjust the position of the latent image.
U.S. Pat. No. 5,175,564 discloses a color printer which senses a mark on a drum and generates a print enable signal which is used to generate the subsequent image lines.
U.S. Pat. No. 4,963,899 discloses a color copier which uses a single imaging station to form successive color images during multiple passes of a photoreceptor belt. Registration marks are formed on the belt and are sensed for purposes of color image registration.
U.S. Pat. No. 5,208,796 discloses a color system which detects the passage of either holes formed in the belt surface or marks formed on the belt surface to generate transverse error correction signals.
Co-pending application, U.S. Ser. No. 07/930,642, filed Aug. 17, 1992, assigned to the same assignee as the present invention, discloses an apparatus and method of positional tracking of a moving photoconductive belt and adjusting an imager in an electrophotographic printing machine to correct for alignment errors when forming a composite image. Registration errors are sensed by developing an appropriate set of target marks, detecting the target marks, and controlling the position of the imager.
For these prior art, mark-detecting systems, the instantaneous position of the mark is sensed. Exact determination of the center of the registration marks within the desired accuracy of plus or minus 5-10 microns is difficult because of the "noise" factor. Noise is attributable to stray light, variations in the registration mark and the distance between the photoreceptor belt and the detector. Also, for those systems using a laser Raster Output Scanner (ROS) as the imager, additional noise is created by asymmetry in beam shape, variation in the intensity of the laser source, and wobble of the rotating polygon.
According to the general principles of the present invention, at least one registration mark is formed in a non-image area of the photoreceptor belt and successively illuminated by an imager (in a preferred embodiment a ROS). Each illumination of the mark is detected by a photodetector to provide output signals representative of the instantaneous position of the mark. These output signals are processed in a signal averaging circuit to enable very precise determination of belt position. Using signal averaging techniques, the accuracy of the mark detection is improved over the accuracy of a single measurement by a factor equal to 1.div..sqroot.n, where n is the number of measurements.
In a first embodiment, a registration mark in the form of a chevron or a "V" mark is formed on one side of the belt in a non-image area and, during each scan, a ROS imager scans across the chevron with a plurality of scans. Light reflected from the mark is detected and each scan produces signals representing a new pair of measured data points. Each measurement is extrapolated using a least squares fitting technique to predict the intersection of the two legs of the chevron with a high degree of precision. A signal is generated which is used for both start of page registration (first scan line of the first image frame) as well as lateral registration (first pixel on the start of each image scan line). In a second embodiment, two chevron marks are formed at opposite sides of the belt in non-image areas; scan line crossings of the mark are then averaged and compared in time to generate skew correction signals. In a third embodiment, a cluster of overlapping chevron marks are formed in a location separate from the previous marks and line scans across the overlapping marks are measured to produce very accurate line-to-line scans within each image frame. More particularly, the present invention relates to an electrographic printer, which incorporates a photoreceptor belt which moves along a preselected path,
imaging means for forming a plurality of registered color images with a plurality of scan lines on the surface of said belt, said color images formed in overlying registration to form a composite color image, PA1 means for forming at least one registration mark on the surface of said belt, PA1 means for sensing a plurality of scan lines crossing said mark, each scan line generating two output signals representing two detection points of said mark, and PA1 means for averaging the signals representing the mark detection and for generating output signals for controlling the operation of the imaging means to register said color images.